--- a/widget/cocoa/nsCocoaWindow.h
+++ b/widget/cocoa/nsCocoaWindow.h
@@ -74,16 +74,25 @@ typedef struct _nsCocoaWindowList {
 - (void)mouseMoved:(NSEvent*)aEvent;
 - (void)updateTrackingArea;
 - (NSView*)trackingAreaView;
 
 - (void)setBeingShown:(BOOL)aValue;
 - (BOOL)isBeingShown;
 - (BOOL)isVisibleOrBeingShown;
 
+// Returns an autoreleased NSArray containing the NSViews that we consider the
+// "contents" of this window. All views in the returned array are subviews of
+// this window's content view. However, the array may not include all of the
+// content view's subviews; concretely, the ToolbarWindow implementation will
+// exclude its TitlebarGradientView from the array that is returned here.
+// In the vast majority of cases, the array will only have a single element:
+// this window's mainChildView.
+- (NSArray<NSView*>*)contentViewContents;
+
 - (ChildView*)mainChildView;
 
 - (NSArray*)titlebarControls;
 
 - (void)setWantsTitleDrawn:(BOOL)aDrawTitle;
 - (BOOL)wantsTitleDrawn;
 
 - (void)setUseBrightTitlebarForeground:(BOOL)aBrightForeground;

--- a/widget/cocoa/nsCocoaWindow.mm
+++ b/widget/cocoa/nsCocoaWindow.mm
@@ -1204,36 +1204,39 @@ void nsCocoaWindow::HideWindowChrome(boo
   // Remove child windows.
   NSArray* childWindows = [mWindow childWindows];
   NSEnumerator* enumerator = [childWindows objectEnumerator];
   NSWindow* child = nil;
   while ((child = [enumerator nextObject])) {
     [mWindow removeChildWindow:child];
   }
 
-  // Remove the content view.
-  NSView* contentView = [mWindow contentView];
-  [contentView retain];
-  [contentView removeFromSuperviewWithoutNeedingDisplay];
+  // Remove the views in the old window's content view.
+  // The NSArray is autoreleased and retains its NSViews.
+  NSArray<NSView*>* contentViewContents = [mWindow contentViewContents];
+  for (NSView* view in contentViewContents) {
+    [view removeFromSuperviewWithoutNeedingDisplay];
+  }
 
   // Save state (like window title).
   NSMutableDictionary* state = [mWindow exportState];
 
   // Recreate the window with the right border style.
   NSRect frameRect = [mWindow frame];
   DestroyNativeWindow();
   nsresult rv = CreateNativeWindow(frameRect, aShouldHide ? eBorderStyle_none : mBorderStyle, true);
   NS_ENSURE_SUCCESS_VOID(rv);
 
   // Re-import state.
   [mWindow importState:state];
 
-  // Reparent the content view.
-  [mWindow setContentView:contentView];
-  [contentView release];
+  // Add the old content view subviews to the new window's content view.
+  for (NSView* view in contentViewContents) {
+    [[mWindow contentView] addSubview:view];
+  }
 
   // Reparent child windows.
   enumerator = [childWindows objectEnumerator];
   while ((child = [enumerator nextObject])) {
     [mWindow addChildWindow:child ordered:NSWindowAbove];
   }
 
   // Show the new window.
@@ -2901,16 +2904,20 @@ static const NSString* kStateCollectionB
   return mBrightTitlebarForeground;
 }
 
 - (NSView*)trackingAreaView {
   NSView* contentView = [self contentView];
   return [contentView superview] ? [contentView superview] : contentView;
 }
 
+- (NSArray<NSView*>*)contentViewContents {
+  return [[[[self contentView] subviews] copy] autorelease];
+}
+
 - (ChildView*)mainChildView {
   NSView* contentView = [self contentView];
   NSView* lastView = [[contentView subviews] lastObject];
   if ([lastView isKindOfClass:[ChildView class]]) {
     return (ChildView*)lastView;
   }
   return nil;
 }
@@ -3219,16 +3226,25 @@ static const NSString* kStateCollectionB
   NS_OBJC_END_TRY_ABORT_BLOCK_NIL;
 }
 
 - (void)dealloc {
   [mTitlebarGradientView release];
   [super dealloc];
 }
 
+- (NSArray<NSView*>*)contentViewContents {
+  NSMutableArray<NSView*>* contents = [[[self contentView] subviews] mutableCopy];
+  if (mTitlebarGradientView) {
+    // Do not include the titlebar gradient view in the returned array.
+    [contents removeObject:mTitlebarGradientView];
+  }
+  return [contents autorelease];
+}
+
 - (void)updateTitlebarGradientViewPresence {
   BOOL needTitlebarView = ![self drawsContentsIntoWindowFrame];
   if (needTitlebarView && !mTitlebarGradientView) {
     mTitlebarGradientView = [[TitlebarGradientView alloc] initWithFrame:[self titlebarRect]];
     mTitlebarGradientView.autoresizingMask = NSViewWidthSizable | NSViewMinYMargin;
     [self.contentView addSubview:mTitlebarGradientView];
   } else if (!needTitlebarView && mTitlebarGradientView) {
     [mTitlebarGradientView removeFromSuperview];